Not Applicable.
Not Applicable
The invention relates to a holder for flat workpieces, particularly semiconductor wafers, particularly in an apparatus for chemico-mechanically polishing the semiconductor.
The miniaturization of semiconductor components which has steadily intensified over the recent years causes more stringent and new demands to the manufacturing process of the electronic components. Thus, the surface of the semiconductor material to be exposed during the lithographic pitting process has to be very flat (the difference in profile being less than 0.4 xcexcm) if the structure sizes are less then 0.5 xcexcm in order to lie within the focussing plane. To this effect, the material needs to be planarized by means of suitable devices.
A process serving the purpose is the Chemical Mechanical Polishing (CM). In this process which uses a polishing agent which is both corrosive and abrasive, the wafer is polished on a polishing clot in plastic at a defined contact force under a rotatory motion of the polishing cloth and the wafer. While the polishing process is under way the polishing agent (a slurry) will flow onto the polishing cloth and form a film between the clot and the wafer. The slurry which is used consists of a chemically offensive solution to which particles such as silica are added in a colloidal suspension.
From DE 195 44 328 or the company document xe2x80x9cCMP Plaster Tool System Planarization Chemical Mechanical Polishingxe2x80x9d published by the Wolters GmbH company in March, 1996, it has been known to provide appropriate stations and devices for such polishing processes. The wafers are retained by holders in processing units and are pressed by them against the polishing working surface. The holders or holding heads are connected to a spindle of a driving machine which is supported to be adjustable in height in order to press the wafer against the working surface. To obtain sufficient planarity, the lower support plate which holds the wafer via vacuum channels or bores is hinged by a universal joint to a carrier portion which, in turn, is connected to the spindle of the driving mechanism. The contact pressure is applied to the support plate via the universal joint.
From DE 197 55 975 A1, it further has become known to guide a support plate for the known holder in a carrier so as to be movable in height and to dispose an annularly closed membrane between the carrier portion and the support plate. The enclosed inner space of the membrane is optionally connected to the atmosphere or a vacuum or a fluid source under pressure. The pressure and vacuum help in displacing the support plate relative to the carrier. In this way, the contact pressure is applied to the support plate on a large surface, which causes an improved result to be obtained in planarization.
Apart from influencing other parameters such as the speed of the wafer, the speed of the polishing disk, the oscillating motions of the polishing head, the supply of polishing agent, and the condition and wear of the polishing cloths, the accuracy and uniformity which can be achieved will have an effect on the result of polishing in the CMP process. Planarized films of 300 mm wafers which are processed by CMP machines frequently present a rotationally symmetric, differentiated surface geometry which is characterized by a heavily polished wafer border, the removal of material is least at a small distance from the wafer border, i.e. 3 mm, and the largest removal of material is achieved about 20 mm from the wafer border.
From EP 0 922 531 A1, it has become known to use membranes of elastomeric material, which are disposed at the underside of the support plate and are pressed against the wafers by means of compressed air, for the described holders for wafers (chucks or chuck plates). This way helps obtain a better compensation of non-uniform areas. The membranes are thin-walled moulded-rubber parts to which compressed air can be applied via bores in the support plate. A holder of this type is constructed as a unit and the contact pressure acting on the wafer during the polishing process is exclusively exerted via the membrane. Apart from performing its function of transmitting the polishing pressure and the torque to the wafer, the holder also has to be capable of lifting the wafer from the polishing disk, thus overcoming adhesion between the wafer and the polishing disk. It is known to realize this operation by producing a negative pressure at the back of the wafer.
A disadvantage in all of the known designs is the fact that the membranes, in turn, are sucked into ring-shaped or sucker-shaped indentations to produce the vacuum required for suction in order to generate chambers having a negative pressure in this way. This causes the membrane to expand to a relatively large extent and to rapidly undergo wear, as a consequence. Moreover, the membrane has to be designed with very thin walls, the disadvantage being that a torque can be transmitted to the wafer only to a low efficiency. Membranes which are known are about 0.5 mm in thickness.
It is the object of the invention to improve a holder provided with a membrane to the effect that it is given a higher stability in standing and may also be employed in a more differentiated manner in order to achieve a geometry of material removal which is as uniform as possible.
The inventive holder relies on a construction in which the support plate is suspended from the carrier portion via a ring-shaped membrane and compressed air, an atmosphere or a negative pressure can be optionally applied to the pressure chamber defined between these components. In this way, the contact pressure of the support plate on the workpiece, particularly on the wafer, can be produced by compressed air and the suspension of the support plate via a universal joint permits the support plate to soundly rest on the workpiece with no danger of chocking. As was stated above a holder of this type is known as such. According to the invention, it is provided with a contact membrane which is appropriately mounted at the border of the support plate in an air-tight way. Thus, the gap between the membrane and the support plate may be pressurized so that the axial force to press the head against the workpiece is produced, on one hand, and a pressure cushion is built up between the support plate and the membrane and provides for an appropriate yielding resilience to exist between the membrane and the workpiece, which reduces non-uniformities in the removal of material. Changing the pressure allows the contact pressure to vary, which achieves a further advantage, in that, if an atmosphere is applied to the pressure chamber the sole force of the weight exerted by the support plate can be used as a polishing force, which leads to a geometry of material removal which differs from the one if compressed air is applied to the membrane.
According to the invention, it is contemplated that the back of the membrane has socket-shaped lugs which are integrally formed to the membrane and extend into bores of the membrane. The socket-shaped lugs are provided with appropriate junctions for connection to a feed line which is located within the pressure chamber and is adapted to be connected, in turn, to a vacuum source. In this way, the negative pressure necessary for holding and, in particular, moving the workpiece is produced directly at the underside of the membrane with no need to deform the membrane for this purpose. Thus, it is unnecessary to expand or upset the membrane to achieve the vacuum required to move the wafer. Therefore, the membrane may be designed with relatively thick walls, e.g. having a thickness of at least 1.5 mm.
The invention helps in adjusting the polishing pressure differently. The force by which the head presses a wafer against the polishing cloth is composed of the weighting force of the support plate and the pressure which is produced in the pressure chamber between the support plate and the carrier portion and which also prevails between the underside of the support plate and the membrane. The differing way of applying forces causes a differing geometry of material removal. This one, for example, is not uniform at all for rigid support plates, but it has rather been shown that an intense removal of material is produced at the border of the wafer and is largely reduced at a slight distance from the border and will increase again towards the centre of the wafer. Naturally, efforts are made to obtain a geometry of material removal which is as uniform as possible. The inventive holder comes closer to this aim.
In an aspect of the invention, the support plate has a circular recess of a small depth which extends nearly up to the border of the support plate. Now, if the diameter of the membrane is chosen so as to coincide with the diameter of the workpiece, e.g. a wafer, the geometries of material removal which ensue therefrom are quite different, depending on whether working is done only by the weighting force of the support plate or by an extra contact pressure because of a positive pressure between the support plate and the carrier section.
According to an aspect of the invention, the socket-shaped lugs of the membrane are provided with junctions which, in turn, are in communication with a manifold line such as a closed-loop line. The manifold line as well as the junctions support themselves on the membrane via the socket-shaped lugs of the membrane. Thus, the manifold line xe2x80x9cfloatsxe2x80x9d in the pressure chamber.